Compensated toner density control for electrostatic photocopies

ABSTRACT

A toner density control for use in electrostatic photocopiers employs a lamp optically coupled to a photosensor over a light path through a latent electrostatic image developer liquid. The spacing between the lamp and photosensor is uniformly, cyclically varied to derive differential photosensor electrical responses indicative of the toner particle concentration or density in the developer liquid. The differential electrical responses are processed and utilized to selectively control replenishment of the toner particle concentration in the developer liquid.

United States Patent DuBois et al.

154] COMPENSATED TONER DENSITY CONTROL FOR ELECTROSTATIC PHOTOCOPIES [72] inventors: Robert Clark DuBois, Fairfield; Donald G. Mikan, Ridgefield; Wayne H. Miller, Stamford, all of Conn.

[ 73] Assignee: Pitney-Bowes, Inc., Stamford, Conn.

[22] Filed: Jan. 21, 1971 [21] Appl. No.: 108,329

[52] US. Cl ..ll8/637, 117/37 LE, l18/DIG. 23, 137/93 [51] Int. Cl. ..G03g 13/00 [58] Field of Search ..118/D1G. 23, 637;.137/93; 117/37 LE; 250/218; 356/201 [56] References Cited UNITED STATES PATENTS 3,369,524 2/1968 Fiihrer 18/637 51 Oct. 17, 1972 3,381,135 4/1968 Keller ..250/218 3,494,328 2/1970 Maloney ..1l8/637 3,368,525 2/1968 Sacre ..1 18/637 3,273,752 9/1966 l-loreczky ..1 18/637 Primary ExaminerMervin Stein Assistant Examiner-Leo Millstein -Email s ewilr-ilb 'b Martin D. Wittstein and Louis A. Tirelli [57] ABSTRACT A toner density control for use in electrostatic photocopiers employs a lamp optically coupled to a photosensor over a light path through a latent electrostatic image developer liquid. The spacing between the lamp and photosensor is uniformly, cyclically varied to derive differential photosensor electrical responses indicative of the toner particle concentration or density in the developer liquid. The differential electrical responses are processed and utilized to selectively control replenishment of the toner particle concentration in the developer liquid.

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INVENTORS ROBERT CLARK DUBOlS DONALD G. MIKAN WAYNE H. MILLER M mm ATTORNEY BACKGROUND OF THE INVENTION In all electrostatic photocopy machines some method is employed at the latent electrostatic image development stage of the copy process for replacing electroscopic toner particles removed with the latent electrostatic image bearing member as it passes through the development stage. A typical practice is to arbitrarily add a predetermined amount of toner particles with each copy cycle or predetermined number of copy cycles. While this technique is quite inexpensive to implement in an automatic photocopier it cannot account for the fact that the amount of toner particles removed during each copy cycle can and in practice does vary over a wide range. That is, different latent electrostatic images require different amounts of toner particles to visibly develop. For example, a simple final image on a large white background requires a small amount of toner particles, while a complex image with very little white background or a light image on a dark background require substantial amounts of toner particles. As a compromise solution to this problem, the amount of toner particles added is typically determined on the basis of the amount required to develop an average image. Nevertheless, it is appreciated that, depending on the character of the images being copied, the toner particle concentration can increase or decrease over a period of time to undesirable levels, giving rise to poor copy quality.

Moreover, it is difficult to insure that equal amounts of toner particles are added each time, thus further complicating the maintenance of adequate toner particle concentration by this method.

To more adequately handle this problem in electrostatic photocopiers employing liquid developer, toner density controls have been devised to monitor the opacity (or conversely the transparency) of the developer liquid which is directly related to the toner particle concentration or density therein. When the opacity of the developer liquid decreases to a predetermined level, additional toner particles, usually concentrated in a liquid suspension form, are added to the liquid developer bath. To monitor the toner particle concentration of the developer bath light is beamed through the developer liquid onto a suitable photosensor, such as a photovoltaic cell, photoresistor or phototransistor.

In practice, such toner density controls unfortunately develop, in time, considerable inaccuracies. Film buildups at the interfaces of the developer liquid with the light source and photosensor affect the photosensor response but have no relation to the current toner particle concentration. Other factors which affect the photosensor response, but are independent of toner particle concentration, include lamp and photosensor aging, lamp voltage variations, photosensor temperature drift, etc.

It is accordingly an object of the present invention to provide an improved toner density control for electrostatic photocopiers employing liquid developer.

An additional object is to provide a toner density control of the above character employing a lamp and photosensor combination for optically monitoring the opacity of the developer liquid and thus the toner particle concentration therein.

Yet another object is to provide a toner density control of the above character wherein the photosensor response is compensated for those factors which are independent of the toner particle concentration of the liquid developer bath.

A further object is to provide a compensated toner density control of the above character which is simple in design, inexpensive to manufacture, reliable, and accurate over a long operating life.

Other objects of the invention will in part be obvious and in part appear hereinafter.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an improved toner density control for monitoring the toner particle concentration in a liquid developer bath included at the development station of an electrostatic photocopier and for initiating the addition of toner particles to the liquid developer bath before the toner particle concentration becomes inadequate to produce good copy. The toner particle concentration is monitored on the basis of the opacity of the developer liquid using a radiation source and a radiation sensor. As an important feature of the present invention, the toner density control is compensated for variations in the sensor response which are unrelated to toner particle concentration. Thus the instant toner density control is more reliable and accurate over a long operating life than prior toner density controls.

To this end, the length of the radiation path through the developer liquid between the source and sensor is uniformly cyclically varied at a rate considerably greater than the rates of change of those variables which are to be compensated. The response of the sensor goes to a maximum value when the radiation path is reduced to a minimum length and decreases to a minimum value as the radiation path is increased to a maximum length during each cycle of path length variation. This alternating maximum and minimum sensor response is superimposed on a relatively constant sensor response due to such factors as aging, drift, temperature, etc. However, the difference between the maximum and minimum sensor responses during each cycle is a direct measure of the opacity of the developer liquid in that portion of the radiation path between its minimum and maximum lengths. This differential sensor response, as contrasted to the absolute values of the maximum and minimum sensor responses, is completely independent of the relatively constant sensor response arising from those factors unrelated to toner particle concentration.

The toner density control of the present invention is adapted to detect the cyclically varying differential electrical response of the sensor which is a measure of the toner particle concentration. When the toner particle concentration is low, there is less attenuation of the radiation over the differential portion of the radiation path and thus there is a small difference in the maximum and minimum values of the sensor response. On the other hand, if the toner particle concentration is high, there is greater differential radiation attenuation and thus a larger differential in the maximum and minimum sensor responses during each cycle of radiation path length variation.

To control the toner particle density of the developer liquid, the apparatus of the present invention is adapted to initiate the addition of toner particles to the developer bath when either the differential between the maximum and minimum sensor responses falls below a predetermined magnitude or a selected ratio based on the sensor differential response departs from a predetermined value.

To obtain the sensor differential response, the spacing between the source and the sensor is uniformly cyclically varied to effectively achieve the requisite variation in radiation path length through the developer liquid. Preferably, either the source or the sensor is fixedly positioned and the other is mounted on a rotary or reciprocating mechanism in order to vary the physical spacing therebetween in cyclical fashion.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a typical electrostatic photocopier liquid developer station adapted with an embodiment of an opacity detector portion of a toner density control in accordance with the invention;

FIG. 2 is a partial sectional view taken along line 2- 2 of FIG. 1;

FIG. 3 is a schematic diagram of one form of processing circuitry for handling the detector electrical output of FIG. 1;

FIG. 4 is a schematic diagram of another form of processing circuitry for handling the detector electrical output; and

FIG. 5 is a detailed circuit schematic diagram of yet another form of detector output processing circuitry coupled with toner concentrate additive control cir cuitry.

Like reference numerals refer to corresponding parts throughout the several figures of the drawings. 7

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring to FIG. 1, a tank 10 contains a developer liquid 12 which consists of electroscopic toner particles suspended in a liquid vehicle. A series of laterally spaced upper guides 14 and a series of laterally spaced lower guides 16 define an arcuate path through the liquid developer bath for a copy sheet 18 hearing on its surface a latent electrostatic image to be visibly developed. As is well understood in the art, electroscopic toner particles suspended in the developer liquid are attracted and held to the charged image areas but not to the uncharged background areas on the surface of copy sheet 18. As the copy sheet exits from the liquid developer bath in tank 10, toner particles in image formation leave with it, thus reducing the toner particle concentration of the liquid developer bath. A certain amount of the liquid vehicle also leaves with the copy sheet, but a large proportion of this is expressed from the copy sheet by squeegy rollers and returned to the liquid developer bath. As a consequence, the toner particle concentration in the liquid developer bath is reduced with each passage of a copy sheet 18 therethrough, and ultimately must be replenished if good copy quality is to be maintained.

A canister 20 containing toner particles suspended in a liquid vehicle, typically referred to in the trade as toner, is inverted with its opening accommodated in a suitable dispenser, generally indicated at 22. This dispenser, whose design may be based on the principles of the well-known chicken feeder, operates to automatically introduce toner from canister 20 into the developer bath so as to maintain the level concentration developer liquid 12 in tank 10 substantially constant. In practice, the toner particle concentration of the toner introduced from canister 20 may be on the order of 0.35 percent, whereas the toner particle concentration in the liquid developer bath is typically on the order of 0.45 percent in order to obtain good copy quality. Thus, in practice, the addition of toner from canister 20 is principally for the purpose of maintaining the level of the developer liquid 12 in tank 10 substantially constant and not to replenish the toner particle concentration in the liquid developer bath.

To replenish the toner particle concentration, a bottle 24 containing a liquid vehicle and a greater toner particle contration than is required in the liquid developer bath is provided. In the trade, there are two forms of toner particle concentrate additives which are commonly used. One form is called intensifier having a toner particle concentration of 2.5 percent. The other form is called replenisher" which typically has a toner particle concentration of 50 percent. As will be seen, the present invention readily operates with either form of toner particle concentrate additive.

Still referring to FIG. 1, the bottle 24 is adapted with a suitable pump, generally indicated at 26, having a discharge spout 28 for conveying the additive into the liquid developer bath contained in tank 10. The actuator element for pump 26, indicated at 30, is depressed by the free end of a pivotally mounted arm 32 which is linked to the armature of a pump solenoid 34. As is apparent from FIG. 1, the application of a current pulse to pump solenoid 34 results in downward movement of arm 32 and depression of the pump actuator 30, and a slug of additive, either intensifier or replenisher is pumped from the bottle 24 into the liquid developer bath 12. As will be seen from the description to follow, it is not necessary that the amount of additive pumped into the liquid developer bath with each actuation of the pump solenoid 34 be metered. Upon the termination of the current pulse applied to pump solenoid 34, a suitable return spring (not shown) returns the pump ac tuator 30, arm 32 and pump solenoid armature 34 to rest positions to await the next current pulse.

As is common practice, an impeller (not shown) imparts a subtle agitation to the developer bath so as to insure uniform distribution of the toner particles throughout the developer liquid.

In accordance with the present invention, monitoring of the toner particle concentration in the developer liquid 12 and the controlled addition of toner particle concentrate from bottle 24 is carried out on the basis of the opacity (or conversely the transparency) of the developer bath. As is understood, the toner particles serve as a pigment or dye which, upon attraction and retention to the latent electrostatic image on the copy sheet 18, renders the image visible. Thus, the concentration of toner particles in the liquid developer bath has a direct relation to the opacity of the developer liquid. As the concentration of toner particles decreases, the developer liquid becomes less opaque or more transparent to light, and vice versa.

To monitor the opacity of the developer liquid 12, a light source in the form of a lamp 40 is submerged in the liquid developer bath and its light output is observed by a suitable photosensor, such as a photovoltaic cell, photoresistor, phototransistor, etc., mounted in the side wall of tank at a point below the developer liquid level. The electrical response of the photosensor 42 is depended in large upon the degree of attenuation of the light from lamp 40 incident on the photosensor by the developer liquid in the light path therebetween. It is understood that the degree of light attenuation is depended upon the opacity of the intervening developer liquid 12.

While the invention is disclosed in terms of using light radiation as the toner particle concentration monitoring energy, it will be appreciated that other forms of radiation susceptible to attenuation in proportion to the concentration of toner particles in the liquid developer bath may be employed.

As an important feature of the present invention, lamp 40 is mounted by a mechanism, generally indicated at 44, which is adapted to vary the physical spacing between the lamp and the photosensor 42 in uniformly cyclical fashion. While mechanism 44 may take a variety of forms, a suitable arrangement disclosed in FIGS. 1 and 2, includes a horizontally oriented disc 46 concentrically mounted at the lower end of a driven shaft 48. The lamp 40 is mounted by the disc 46 at a point displaced from the axis of shaft 48 such that, upon rotation of the disc, the lamp is orbited in a circular path from a solid line position in FIG. 2 remotely spaced from the photosensor 42 around to a phantom line position 40' relatively closely spaced to the photosensor and back around to the remotely spaced position. A suitable slip ring arrangement (not shown) may be used to bring electrical power to lamp 40.

It is seen, especially in FIG. 2, that when the lamp 40 is in the solid line position, the optical path through the developer 12 to the sensor 42 is considerably longer than it is when the lamp is in its phantom line position 40. With rotation of the disc 46, the optical path length goes through a minimum and a maximum value with each shaft revolution or operating cycle of mechanism 44.

When the lamp 40 arrives at its position of maximum distance from the photosensor 42, the electrical response of the photosensor to incident light is a minimum for the existing toner particle concentration. The electrical response of the photosensor 42 is a maximum when the lamp assumes its position of closest proximity thereto. The alternating maximum and minimum electrical responses of the photosensor 42 occur at the same cycling rate as that of the harmonic motion of lamp 40. It is seen that the differential between the maximum and minimum photosensor responses is proportional to the attenuation of the lamp light output over an optical path having a length equal to the difference between the maximum and minimum 6 optical path lengths between the lamp and photosensor 42; this differential path length being equal to the diameter of the orbital lamp path.

The relative maximum and minimum photosensor electrical responses, as contrasted to the absolute values thereof, are indicative of toner particle concentration, but are independent of any spurious factors affecting photosensor response which are of a relatively constant nature as compared to the cycling rate of mechanism 44. Specifically, over a period of time a film invariably builds up on the face of the photosensor and the surface of the lamp in contact with the developer liquid. This relatively constant light attenuating film in the optical path between the lamp and the photosensor has no relation to the current toner particle concentration in the liquid developer bath. While the light attenuation introduced by this film has an effect on the absolute values of the photosensor response, it can be effectively canceled out if only the varying or AC component of the photosensor electrical response arising from the motion of lamp 40 is considered. By the same token, other relatively steady state factors affecting a relatively constant or DC photosensor electrical response component, but having no connection or relation to the toner particle concentration in the developer liquid 12, are lamp and photosensor aging, variations in the lamp energizing voltage, temperature drift of the photosensor and associated circuitry, etc. As in the case of film build-up, these factors proportionately effect the photosensor response regardless of the relative lamp position. Thus, by considering only the differential or AC response of the photosensor during each lamp cycle, the effects of these factors may be compensated. It is thus seen that the toner density control of the present invention can be readily and fully compensated for these factors by producing cyclical motion of the lamp relative to the photosensor and considering-the alternating or AC component of the electrical response of the photosensor.

At this point, it should be noted that the requisite AC response of the photosensor can be achieved by reversing the positions of lamp 40 and photosensor 42 shown in FIGS. 1 and 2.

The circuit of FIG. 3 is particularly adapted to processing the photosensor differential electrical response due to the lamp cyclical motion in a manner such as to provide an output signal which is proportional to the toner particle concentration in the liquid developer bath and completely independent of those factors unrelated to toner particle concentration. The output control signal is used, in practice, to initiate and control the selective application of current pulses to pump solenoid 34 when the toner particle concentration falls below a predetermined level and thereby effects the addition of toner particle concentrate from bottle 24 (FIG. 1). The photosensor 42 in FIG. 3 is shown to be a photoresistor, however it will occur to those skilled in the art that photovoltaic cells or phototransistors may be implemented instead.

As seen in FIG. 3, the photosensor 42 is connected across a DC supply 50 and a series resistor R1. The varying photosensor electrical response due to the cyclically varying intensity of the light incident on it from the orbiting lamp 40 (FIG. 1) is in the form of a varying DC voltage developed across the photosensor 42. This varying voltage is applied to opposite poles of a double pole, double throw switch S1. The contacts of switch Sl are cross-connected such that the voltage across photosensor 42 may be applied to upper and lower halves, 51 and 52, respectively, of a ratio computing network, generally indicated at 53. Specifically, as seen in FIG. 3, when the photosensor output voltage is applied to the upper half 51 of the ratio computing network 53, the lower half 52 is connected to ground, and vice versa. Switch S1 is operated in synchronism with the orbital movement of lamp 40, and in practice would be mechanically coupled with mechanism 44 (FIG. 1). Thus, in application, the ratio computing network 53 samples the voltage across photosensor 42 for two positions, 180 apart, of lamp 40 during each orbital cycle. While not essential, the most logical times to sample the effective photosensor electrical output is when the lamp assumes its positions of maximum and minimum spacing relative to the photosensor.

For purposes of the present description, it is assumed that switch S1 is operated such that the maximum photosensor output voltage V is applied to the upper half 51 and the minimum photosensor output voltage V,,,,,, is applied to the lower half 52 of the ratio computing network 53 during each lamp cycle. Input V is in the form of discrete voltage pulses occurring at the cycling rate of the lamp; the pulses being integrated by the circuit combination of resistor R2 and capacitor Cl connected at the input ofa reciprocal amplifier 54. The output of this amplifier is a voltage proportional to the reciprocal of V labeled l/V in FIG. 3, and is applied as one input to a multiplying amplifier 56. Reciprocal and multiplying amplifiers are well known components used in analog computing systems.

Input V also in the form of a series of voltage pulses occurring at the lamp cycling rate, is integrated in an integrating circuit, consisting of resistor R3 and capacitor C2, and applied as the second input to multiplying amplifier 56. It is seen that the output of amplifier 56, indicated V /V is in the form of a ratio of the minimum photosensor response to the maximum photosensor response. It is seen that if the differential between V and V,,,,,, decreases, indicating a lowering of the toner particle concentration in the liquid developer bath, the ratio output signal V /V increases. It will readily be appreciated by those in the art that a suitable threshold detector and intensifier pump control, such as specifically disclosed in FIG. 5, may be provided to handle the ratio output voltage V,,,,,,/V,,,,, from amplifier 56. When the ratio output voltage rises above a predetermined amplitude established by the threshold detector, pump 26 (FIG. 1) is activated to add toner particle concentrate.

In the alternative embodiment of FIG. 4, the cyclically varying voltage developed across photosensor 42 in response to the cyclical motion of lamp 40 is supplied to a ratio computing network, generally indicated at 58. Since the circuit of FIG. 4 does not employ the switch 811 of FIG. 3, the input to the ratio computing network 58 is in the form of a rippling DC voltage having a ripple rate equal to the lamp cycling rate. Another way of considering this photosensor output voltage is that it has two components, namely, an AC component (ripple) superimposed on a DC component which includes the effects of film build-up, temperature drift, voltage fluctuations, etc.

The rippling DC voltage (AC plus DC components) passes through a diode D1 and is integrated in an integrating circuit consisting of resistor R3 and capacitor C2. The output of this integrating circuit is supplied as one input V to multiplying amplifier 56. Diode D1 is poled such as to prevent capacitor C2 from discharging back into the photosensor circuit as the rippling DC voltage goes through the valleys between its peaks.

The rippling DC voltage developed across photosensor 42 is also applied to a series capacitor C3. This capacitor is effective to pass the ripple or AC component, but rejects the DC component of the photosensor output voltage. The AC component is half-wave rectified by a series diode D3 and integrated in an integrating circuit consisting of resistor R2 and capacitor C1 to provide a DC voltage signal [V,,,,, V,,,,,,]/2. The factor 2 arises from the fact that the AC component passed by capacitor C3 is symmetrical about ground and is then half-wave rectified by diode D2. Thus, amplifier 54 sees only one-half of the peak to peak voltage of the AC component; the peak to peak amplitude being V V The output of reciprocal amplifier 54, 2/V,,,,,,-V,,,,,,, is multiplied with V by multiplying amplifier 56 to achieve the ratio output 2V,,,,,,/[ maI mtnIi- As in the case of the circuit of FIG. 3, when the toner particle concentration decreases, the differential between V and V,,.,,, decreases, and thus the ratio output voltage 2V ,/[V,,,,, V increases in magnitude. This ratio output voltage may be handled in the same manner as the ratio output voltage V /V of FIG. 3 to appropriately initiate and control the addition of toner particle concentrate. It will be appreciated that the circuits of FIGS. 3 and 4 may be modified such as to achieve an inversion of the output ratios indicated, in which case the ratio output voltages would decrease as toner particle concentration decreases. The addition of toner particle concentrate would be initiated, in this situation, when the ratio output voltage magnitude falls below a predetermined threshold level.

It can be shown that a suitable ratio based on the differential photosensor response to cyclical variation in the spacing between the lamp and photosensor, such as ratios V /V and 2V ,/[V,,,,, V derived from the circuits of 3 and 4, respectively, are proportional to the toner particle concentration in the liquid developer bath and completely independent of such factors as film build-up, temperature drift, voltage variation, photosensor and lamp aging, etc. It will be appreciated that if, for example, the lamp voltage decreases, there will be a proportionate decrease in the magnitudes of the maximum and minimum photosensor responses, as well as the differential therebetween. Thus, if the lamp voltage decreases by 10 percent as often occurs in practice unless it is fed from a regulated supply, the maximum and minimum photosensor responses (V and V decrease by a certain amount, such as 30 percent. It will be seen that, in the ratio outputs of FIGS. 3 and 4, the percentage decrease in the mag nitudes of V and V effectively cancel out. This will also be true for the factors of film build-up, temperature drift, etc., and thus the ratio outputs of FIGS. 3 and 4 are completely independent thereof and fully compensated therefor.

It will occur to those skilled in the art that other ratios based on the differential photosensor response to cyclical variation in the lamp-photosensor spacing and other circuits for obtaining outputs proportional thereto, as well as other circuits for achieving the ratio outputs of FIGS. 3 and 4, may be provided without departing from the teachings of the present invention.

A less perfect, but more economical technique for processing the differential photosensor electrical response, due to the costliness of the analog computing elements 54 and 56 of FIGS. 3 and 4, is to control toner particle concentration replenishment on the basis of the differential between the photosensor maximum and minimum responses in a manner such as illustrated in FIG. 5. Referring to FIG. 5, photosensor 42 is illustrated as a photovoltaic cell, which generates a voltage proportional in magnitude to the amount of light incident on it from lamp 40 during its orbital movement. The output voltage of the photovoltaic cell, developed across resistor R5, includes, as previously noted, an AC component whose frequency is equal to the orbital rate of lamp 40 and a DC component. The AC voltage component is coupled by a series capacitor C5 across resistor R6 for application to the input of a low frequency, proportional amplifier 60. Series capacitor C5 blocks the DC component of the photosensor output voltage from the amplifier input.

The amplifier AC voltage component at the output of amplifier 60 is half-wave rectified by diode D5 and integrated in an integrating network consisting of resistor R7 and capacitor C6. Capacitor C6 is shunted by a resistor R8 and an operating coil L1 for a relay K1. The values of resistor R7 and capacitor C6 are chosen to give a short time constant in relation to the frequency of the AC voltage component, which may be on the order of lHz. Thus, capacitor C6 will rapidly charge to a voltage level equal to the amplitude of the pulses passed by diode D5 (one-half the AC component peak to peak voltage amplitude). If the amplitudes of the voltage pulses are increasing, indicating an increase in the toner particle concentration, the voltage across capacitor C6 can follow along closely. Resistor R8, on the other hand, is chosen to be substantially larger than resistor R7 so as to provide a relatively long time constant with capacitor C6, and thus to hold the charge on capacitor C6 substantially constant so long as the voltage pulse amplitude remains substantially constant while permitting a gradual discharge thereof, if the voltage pulses are decreasing in amplitude, which indicates a reduction in the toner particle concentration.

As long as the voltage capacitor C6 remains above a predetermined level corresponding to the in voltage of relay Kl, its plunger 62 opens normally closed relay contacts 64. When the voltage across capacitor C6 decreases to a level corresponding to the drop-out voltage of relay K1, which is slightly below its pull-in voltage, plunger 64 is released to engage contacts 64.

Relay K1, as seen in FIG. 5, operates to selectively apply the energizing electrical power of a battery 66 across an asymmetrical, astable, multivibrator, generally indicated at 70. Multivibrator 70, in turn, operates to periodically energize a relay K2 whose plunger 72 acts to close normally open relay contacts 74 and connect an AC source 76 to the pump solenoid 34. The multivibrator 70 may be adjusted to provide, for example, a thirty second duty cycle for relay K2 10 wherein it is energized for five seconds and de-energized for 25 seconds as long as relay K1 remains de energized to connect battery 66 across the multivibrator circuit. It is thus seen that the pump solenoid 34 receives a 5 second AC pulse from source 76 every 30 seconds to effect replenishment of the liquid developer bath with toner particle concentrate from bottle 24 (FIG. 1).

The operation of the multivibrator is essentially as follows. When relay Kl drops out to close contacts 64, signaling a low toner particle concentration in the liquid developer bath, battery 66 supplies charging current through resistor R9 and resistor R10 to capacitor C8. Resistor R10 is adjustable to vary the duty cycle time for relay K2.- A Zener diode D6, connected across resistor R10 and capacitor C8 establishes a constant voltage toward which the capacitor C8 charges. The junction between resistor R10 and capacitor C8 is connected to the emitter of a unijunction transistor 01. One base terminal of unijunction transistor 01 is connected by a resistor R11 to the junction between resistor R9 and R10, while the other base terminal is connected by a resistor R12 to the negative side of battery 66.

When capacitor C8 charges to a predetermined percentage of the voltage across the base terminals of the unijunction transistor Q1, the unijunction transistor breaks down to develop a positive going pulse across resistor R12 sufficient to fire a silicon controlled rectifier D7. With silicon controlled rectifier D7 now conductive, the operating coil L2 of relay K2 is energized from battery 66 through resistor R13. A diode D8 connected across relay coil L2 is poled to short out back emf generated by the relay coil during current switching.

Relay K2, thus energized, closes normally open contacts 74, as previously described, and also closes normally open contacts 77 to thereby connect battery 66 to the other half of the multivibrator circuit 70. It is seen that with relay contacts 77 closed, a capacitor C9 is charged from battery 66 through resistors R13, R14 and R15. Zener diode D9 establishes a constant voltage toward which capacitor C9 charges and the resistor R15 is variable so as to adjust the on-time of relay K2. The junction between resistor R15 and capacitor C9 is connected to the emitter of a second unijunction transistor Q2, whose one base terminal is connected to the junction between resistors R14 and R15 by a resistor R16 and whose other base terminal is connected to the negative side of battery 66 through a resistor R17.

When capacitor C9 charges to a predetermined percentage of the voltage across the base terminal of unijunction transistor Q2, it breaks down to fire a second silicon controlled rectifier D10. When this control rectifier is in its conductive state, it effectively shorts out the operating coil L2 of relay K2. The thus de-energized relay K2 drops out, opening contacts 74 and 77. Pump solenoid 34 is thus disconnected from its energizing source 76 and the operating cycle of the multivibrator circuit 70 repeats at, for example, thirty second intervals as long as relay K1 is de-energized,

signaling a low toner particle concentration in the liquid developer bath.

llll

It will be observed that the multivibrator circuit 70 may be adapted to handle the ratio outputs of FIGS. 3 and 4, merely by having the contacts 64 of relay Kl normally open rather than normally closed.

The circuitry of FIG. 5 is merely illustrative of one way in which the cyclically varying electrical response of the photosensor can be processed pursuant to achieving selective additions of toner particle concentrate. As an alternative approach, the cyclically varying photosensor electrical response arising from the cyclical motion of the lamp relative to the photosensor (or conversely the photosensor relative to the lamp) can be used to selectively engage a clutch to supply drive to a timing cam which, in turn, acts to periodically depress the pump actuator 30 (FIG. 1) and effect replenishment of the toner particle concentration. Other approaches, both mechanical and electrical in nature, will readily occur to those skilled in the art.

Rather than continuously monitoring the lamp output throughout each cycle, it is also contemplated that a suitable shuttering or strobing technique can be employed to sample the lamp output at two different times during each cycle, such as at the points of maximum and minimum spacings. The corresponding two photosensor electrical responses, such as developed in the circuit of FIG. 3, would be supplied to a differential operational amplifier, the difference output being used to control the actuation of pump solenoid 34.

While the magnitude of the photosensor differential response, as processed by the circuitry of FIG. 5, is somewhat influenced by film build-up, voltage variations, etc., the magnitude of the effect is significantly less than would be the case if toner density control were predicated on the absolute value of the photosensor response, as has been done in the past.

It is also contemplated that instead of cyclically varying the physical spacing between the lamp and photosensor, the same effect of cyclically varying the optical path length therebetween can be achieved using rotating mirrors and the like.

Moreover, the present invention has general application to monitoring the opacity of any fluid medium reliably and accurately for a variety of purposes, not necessarily concerned with controlling or regulating the composition of the fluid medium. For example, the principles of the present invention have applications to chemical manufacturing processes for monitoring mixing and batching operations, smoke control, sewage treatment, etc. Also, selected types of sources, sensors, and/or filters may be employed to limit the energy wavelength to which the system responds, thereby making the system more sensitive to certain properties or characteristics of a fluid and relatively insensitive to others.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having described the invention, what is claimed as new and desire to secure by Letters Patent is:

ll. In an electrostatic photocopier having a development station including a bath of liquid developer through which a latent electrostatic image bearing member is passed and a supply of toner particle concentrate for replenishing the toner particle concentration of the liquid developer bath, a toner density con trol comprising, in combination:

A. a radiation source;

B. a radiation sensor coupled with said source over a radiation path through the liquid developer;

C. means for varying the effective length of said radiation path between said source and said sensor in uniformly cyclical fashion, and

D. means responsive to the varying electrical response of said sensor due to the variation in the radiation path length for selectively controlling the addition of toner particle concentrate from the supply into the liquid developer bath.

2. The toner density control defined in claim 1 wherein said radiation path length varying means operates to cyclically vary the physical spacing between said source and said sensor.

3. The toner density control defined in claim 2 wherein 1. said sensor is a photosensor having a first electrical response when closely spaced relative to said, source and a second, lesser electrical response when remotely spaced relative to said source during each cycle of said radiation path length varying means, said first and second responses being superimposed on a relatively constant photosensor response, and

2. said responsive means operating to reject said constant photosensor response and control the addition of toner particle concentrate on the basis of the differential between said first and second photosensor responses.

4. The toner density control defined in claim 2, wherein said sensor provides a first electrical response when closely spaced relative to said source and a second, lesser electrical response when remotely spaced relative to said source during each cycle of said radiation path length varying means, said responsive means including means for deriving a signal proportional to the ratio of the magnitudes of said first and second responses for controlling the addition of toner particle concentrate.

5. The toner density control defined in claim 4, which further includes means operating in synchronism with said path length varying means for periodically sampling the electrical response of said sensor to provide said first and second responses.

6. The toner density control defined in claim 2 wherein said sensor provides a rippling DC electrical response signal due to the cyclical variation in the physical spacing between said source and said sensor, said responsive means including means for deriving a signal proportional to the ratio of the amplitude of said electrical response signal and the peak to peak amplitude of the ripple portion thereof.

7. The toner density control defined in claim 2 wherein said radiation path length varying means is adapted to mount one of said sensor and said source for orbital movement relative to the other.

8. The toner density control defined in claim 1 wherein said responsive means includes A. a pump for pumping toner particle concentrate from the supply into the liquid developer bath;

B. means for periodically actuating said pump, and

C. means establishing a threshold signal level and operating to activate said actuating means until an electrical signal derived from the varying electrical response achieves a predetermined relationship to said threshold signal level.

9. In an electrostatic photocopier having a development station including a bath of liquid developer through which a latent electrostatic image bearing sheet is passed and a supply of toner particle concentrate for replenishing the toner particle concentration of the liquid developer bath, a toner density control comprising, in combination:

A. a light source;

B. a photosensor coupled with said source over a radiation path through the liquid developer bath;

C. means for uniformly, cyclically varying the relative physical spacing between said source and said photosensor; and

D. means for processing the varying electrical response of said photosensor to said cyclical variation of the spacing between it and said source, said processing means operating to selectively control replenishment of the toner particle concentration in the developer bath from the toner particle concentrate supply.

10. The toner density control defined in claim 9 wherein said processing means includes means for deriving an electrical control signal proportional to the differential electrical response of said photosensor to the cyclical variation in the spacing between it and said source for controlling replenishment of the toner particle concentration.

11. The toner density control defined in claim 10 wherein said differential electrical response is in the form of an AC signal, said AC signal being superimposed on a DC signal, said processing means including 1. an amplifier,

2. a series capacitor operating to couple said AC signal to said amplifier. and block said DC signal,

3. a rectifier for rectifying the output of said amplifier,

4. an integrating network connected to said rectifier for integrating the rectified output of said amplifier to derive said control signal, and

5. a threshold detector for detecting the amplitude of said control signal and initiating toner particle concentration replenishment when said control signal amplitude falls below a predetermined threshold level.

12. The toner density control defined in claim 11, which further includes a pump for pumping toner particle concentrate from the supply into the liquid developer bath, and said processing means further includes means activated from said threshold detector to periodically actuate said pump as long as said control signal amplitude remains below said threshold level. 5 13. The toner density control defined in claim 9 wherein said photosensor provides a rippling DC electrical response to the variation in the spacing between it and said source, and said processing means includes a ratio computing network for deriving a replenishment control signal proportional to the ratio of the maximum and minimum applitudes of said rippling DC electrical 4. The toner density control defined in claim 9 wherein said photosensor provides alternate first electrical responses each time it is closely spaced relative to said source and second,lesser electrical responses each time it is remotely spaced relative thereto, said processing means including means for deriving a replenishment control signal proportional to the ratio of said first and second responses.

15. The toner density control defined in claim 14, which further includes means operating in synchronism with said means for varying the spacing between said sensor and said source to periodically sample the electrical response of said photosensor and provide said first and second responses.

16. The toner density control defined in claim 15 wherein said control signal deriving means includes a reciprocal amplifier receiving one of said first and second responses and a multiplying amplifier for multiplying the output of said reciprocal amplifier with the other of said first and second responses.

17. The toner density control defined in claim 9 wherein said photosensor varying electrical response includes an AC signal superimposed on a DC signal level, said processing means including a ratio computing network for deriving a replenishment control signal proportional to the ratio of the peak amplitude of said varying electrical response and the peak to peak amplitude of said AC signal.

18. The toner density control defined in claim 17 wherein said ratio computing network includes A. a reciprocal amplifier;

B. a multiplying amplifier having 1. a first input connected to the output of said reciprocal amplifier, and 2. a second input;

C. a first circuit including, in series, a capacitor, diode, and integrator connecting said photosensor to said reciprocal amplifier, and;

D. a second circuit including, in series, a diode and integrator connecting said photosensor to said second input of said multiplying amplifier;

E. whereby to provide said replenishment control signal at the output of said multiplying amplifier. 

1. In an electrostatic photocopier having a development station including a bath of liquid developer through which a latent electrostatic image bearing member is passed and a supply of toner particle concentrate for replenishing the toner particle concentration of the liquid developer bath, a toner density control comprising, in combination: A. a radiation source; B. a radiation sensor coupled with said source over a radiation path through the liquid developer; C. means for varying the effective length of said radiation path between said source and said sensor in uniformly cyclical fashion, and D. means responsive to the varying electrical response of said sensor due to the variation in the radiation path length for selectively controlling the addition of toner particle concentrate from the supply into the liquid developer bath.
 2. The toner density control defined in claim 1 wherein said radiation path length varying means operates to cyclically vary The physical spacing between said source and said sensor.
 2. a second input; C. a first circuit including, in series, a capacitor, diode, and integrator connecting said photosensor to said reciprocal amplifier, and; D. a second circuit including, in series, a diode and integrator connecting said photosensor to said second input of said multiplying amplifier; E. whereby to provide said replenishment control signal at the output of said multiplying amplifier.
 2. said responsive means operating to reject said constant photosensor response and control the addition of toner particle concentrate on the basis of the differential between said first and second photosensor responses.
 2. a series capacitor operating to couple said AC signal to said amplifier and block said DC signal,
 3. a rectifier for rectifying the output of said amplifier,
 3. The toner density control defined in claim 2 wherein
 4. an integrating network connected to said rectifier for integrating the rectified output of said amplifier to derive said control signal, and
 4. The toner density control defined in claim 2, wherein said sensor provides a first electrical response when closely spaced relative to said source and a second, lesser electrical response when remotely spaced relative to said source during each cycle of said radiation path length varying means, said responsive means including means for deriving a signal proportional to the ratio of the magnitudes of said first and second responses for controlling the addition of toner particle concentrate.
 5. The toner density control defined in claim 4, which further includes means operating in synchronism with said path length varying means for periodically sampling the electrical response of said sensor to provide said first and second responses.
 5. a threshold detector for detecting the amplitude of said control signal and initiating toner particle concentration replenishment when said control signal amplitude falls below a predetermined threshold level.
 6. The toner density control defined in claim 2 wherein said sensor provides a rippling DC electrical response signal due to the cyclical variation in the physical spacing between said source and said sensor, said responsive means including means for deriving a signal proportional to the ratio of the amplitude of said electrical response signal and the peak to peak amplitude of the ripple portion thereof.
 7. The toner density control defined in claim 2 wherein said radiation path length varying means is adapted to mount one of said sensor and said source for orbital movement relative to the other.
 8. The toner density control defined in claim 1 wherein said responsive means includes A. a pump for pumping toner particle concentrate from the supply into the liquid developer bath; B. means for periodically actuating said pump, and C. means establishing a threshold signal level and operating to activate said actuating means until an electrical signal derived from the varying electrical response achieves a predetermined relationship to said threshold signal level.
 9. In an electrostatic photocopier having a development station including a bath of liquid developer through which a latent electrostatic image bearing sheet is passed and a supply of toner particle concentrate for replenishing the toner particle concentration of the liquid developer bath, a toner density control comprising, in combination: A. a light source; B. a photosensor coupled with said source over a radiation path through the liquid developer bath; C. means for uniformly, cyclically varying the relative physical spacing between said source and said photosensor; and D. means for processing the varying electrical response of said photosensor to said cyclical variation of the spacing between it and said source, said processing means operating to selectively control replenishment of the toner particle concentration in the developer bath from the toner particle concentrate supply.
 10. The toner density control defined in claim 9 wherein said processing means includes means for deriving an electrical control signal proportional to the differential electrical response of said photosensor to the cyclical variation in the spacing between it and said source for controlling replenishment of the toner particle concentration.
 11. The toner density control defined in claim 10 wherein said differential electrical response is in the form of an AC signal, said AC signal being superimposed on a DC signal, said proceSsing means including
 12. The toner density control defined in claim 11, which further includes a pump for pumping toner particle concentrate from the supply into the liquid developer bath, and said processing means further includes means activated from said threshold detector to periodically actuate said pump as long as said control signal amplitude remains below said threshold level.
 13. The toner density control defined in claim 9 wherein said photosensor provides a rippling DC electrical response to the variation in the spacing between it and said source, and said processing means includes a ratio computing network for deriving a replenishment control signal proportional to the ratio of the maximum and minimum applitudes of said rippling DC electrical response.
 14. The toner density control defined in claim 9 wherein said photosensor provides alternate first electrical responses each time it is closely spaced relative to said source and second, lesser electrical responses each time it is remotely spaced relative thereto, said processing means including means for deriving a replenishment control signal proportional to the ratio of said first and second responses.
 15. The toner density control defined in claim 14, which further includes means operating in synchronism with said means for varying the spacing between said sensor and said source to periodically sample the electrical response of said photosensor and provide said first and second responses.
 16. The toner density control defined in claim 15 wherein said control signal deriving means includes a reciprocal amplifier receiving one of said first and second responses and a multiplying amplifier for multiplying the output of said reciprocal amplifier with the other of said first and second responses.
 17. The toner density control defined in claim 9 wherein said photosensor varying electrical response includes an AC signal superimposed on a DC signal level, said processing means including a ratio computing network for deriving a replenishment control signal proportional to the ratio of the peak amplitude of said varying electrical response and the peak to peak amplitude of said AC signal.
 18. The toner density control defined in claim 17 wherein said ratio computing network includes A. a reciprocal amplifier; B. a multiplying amplifier having 